If the status of a battery, for example, a lead battery of a motor vehicle, is to be externally determined without intervention in the battery, the externally accessible variables of current, voltage, and temperature are engaged. These three variables are used as input variables for a method which ascertains the battery status therefrom employing data processing devices. For the current measurement, systems heretofore have used either shunt resistors or inductive current measuring principles, the analysis electronics and the current sensor being integrated into a mechanism. In the case of a lead battery of a motor vehicle, for example, having a current sensor, a voltage detector, a temperature detector, a signal processing and preprocessing unit, and a housing, the entire system is typically integrated into a pole recess of the battery, a terminal protruding from the entire system being electrically connected to the pole. In particular, if operating states are detected directly at the battery, it is necessary to provide the sensor on the battery; furthermore, it is typical to also attach the associated signal preprocessing unit there to prevent corruption due to long lines.
For example, if the current is to be measured using a shunt resistor, the shunt is to be provided directly on the battery to prevent line losses, and it is further typical to also provide a signal preprocessing unit directly on the shunt, in order to keep corruption due to voltage drops and due to voltages induced in the measuring lines as small as possible in the case of low currents, for example, of less than 1 ampere. In order to keep the overall size and the amount of waste heat small, shunt resistors are typically provided with very small resistance values, i.e., <1 ohm or 100 μΩ in the automotive field, for example, so that in the case of small currents, according to U=R×I, also very small voltages of a few nV or μV, which reflect the current level, drop across the shunt resistor. Such small voltage values also occur in the case of temperature detection, since temperature sensors, as a function of the sensor type, also output signals which may easily be corrupted by long cable lengths and because of the low voltage level. In particular, it is to be noted here that in the case of use in an engine compartment of a motor vehicle, numerous interference sources act on the signal cable.
Therefore, conventional battery monitoring systems provide for attaching not only the sensors themselves, but rather also a preprocessing circuit or a detection circuit directly to the battery. The influence of interference sources is thus reduced by preprocessing directly at the sensor.
These systems have the disadvantage of a relatively complex mechanism which is capable of connecting electronic components and current-conducting conductors/sensors in such a way that sufficient corrosion protection and established protective classes, for example, IP5x5 or higher, are ensured, in particular in applications in the automotive field. Furthermore, it is necessary in the conventional system for electronics and signal processing components to be provided in the sensor unit, i.e., on the battery itself, such components typically already being provided in a higher-order control unit or data processing circuit, however, and also being able to be used. For this reason, the components provided for signal preprocessing are provided on the battery and in a higher-order data processing circuit on the basis of the conventional system architecture. Unnecessary costs therefore arise due to the duplicate design of the components.
Attaching a sensor element and a temperature measuring element in a housing of a sensor module, which is provided on the battery, is described in German Published Patent Application No. 102004003198. The sensor module does not contain active electronics, the entire analysis unit being provided in the calculation unit, which contains all further analysis components, i.e., in addition to a calculation unit, also a signal preprocessing unit. A shunt resistor having low resistance, for example, 100 μΩ, is used for the current detection to measure currents between a few milliamperes and kiloamperes. Particularly simple components may be used if a shunt resistor is used as the current sensor element. However, in the above-mentioned document, signal lines, which are susceptible to interference, transmit signals between the sensor module and the combined calculation unit, which is located remotely therefrom. These lines transmit signals having a low level over a distance of up to 10 m in the engine compartment, the supply lines for the combined calculation unit also having to bridge several meters, whereby a ground offset between the negative pole of the battery, on the one hand, and the supply voltage of the calculation unit, on the other hand, results overall, which corrupts the measuring signals. In particular in the case of detection of small currents using a low-resistance shunt, the current intensities which are thus measured are substantially corrupted; this is also true for temperature measuring signals, which have a low voltage level. The system disclosed in German Published Patent Application No. 102004003198 explicitly uses a shared reference voltage source for a shared voltage supply of all components of the calculation unit. The error thus arising due to voltage offset because of the conduction of the supply line or through induced interfering signals, which result due to the different voltage potentials and the superindirect connection of the two supply grounds, is not discussed in this document.